Abstract

In this study, aluminum oxide was deposited on an Al alloy substrate to produce hard ceramic coatings using a plasmaelectrolytic oxidation (PEO) process working at atmospheric pressure. The process utilizes dc and unipolar pulsed dc in the frequency range 0.2–20 kHz. Optical emission spectroscopy was employed to study the species and electron temperature of the plasma. The morphology and microstructure of the coatings were investigated using scanning electron microscopy. It was found that in the first 12 min of the PEO process, the plasma electron temperature increased with the applied voltage during the experiments, the plasma electron temperature was found to be in the range 4000–9000 K, and the applied voltage to the electrodes ranged up to 550–600 V for the different current modes. The plasma temperature profile exhibits a wider peak temperature spike for the dc power mode than for the pulsed dc mode, indicating that the dc plasmadischarges would provide longer sintering time. The pulsed dc mode increases the spike temperature up to 8700 K but does not necessarily enhance the coating growth. The high spike temperature generated by strong discharges likely melts the oxide and then traps gas into the melt pool, resulting in some porosity at the interface. By eliminating the high temperature spike, a denser interface layer and homogenous coating morphology are produced.

Received 15 October 2009Accepted 04 January 2010Published online 29 June 2010

Acknowledgments:

This research was supported by the Natural Science and Engineering Research Council of Canada (NSERC). The Canadian Foundation for Innovation (CFI) is acknowledged for its funding support to acquire the research instrumentation used in this study.